This application relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to a disc drive actuator assembly with improved inertial and vibration characteristics.
Computers commonly use hard disc drives to store large amounts of data in a form that can be readily accessed by a user. A disc drive generally includes a stack of vertically spaced magnetic discs that are rotated at a constant high speed by a spindle motor. The surface of each disc is divided into a series of concentric, radially spaced data tracks in which data are stored in the form of magnetic flux transitions. Typically, each data track is divided into a number of data sectors that store data blocks of a fixed size.
Data are stored and accessed on the discs by an array of read/write transducers (heads) mounted to a rotary actuator. Typically, the actuator includes one or more head support arms which project outwardly from a portion of the actuator that pivots about a shaft secured to a base deck of the disc drive. The stacked discs and head support arms are configured so that the surfaces of the stacked discs are accessible to the heads mounted on the complementary stack of head support arms. Head conductors included on the actuator assembly conduct electrical signals from the heads to a flex circuit, which in turn conducts the electrical signals to a printed circuit board (PCB).
The actuator assembly is mounted disc drive at a position closely adjacent the outer diameter of the discs. The actuator assembly includes a coil which, as part of a voice coil motor (VCM), causes the head support arms and the heads to be pivotally moved about a shaft. Thus, the head support arms move in a plane parallel to the surfaces of the discs to position a head over a selected data track.
The VCM includes a coil mounted radially outward from the cartridge assembly, the coil being immersed in a magnetic field of a magnetic circuit of the VCM. The magnetic circuit includes one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit so that the coil moves in accordance with well-known Lorentz relationship. As the coil moves, the actuator assembly pivots about the shaft and the heads move across the disc surfaces.
Each of the heads is mounted to a head support arm by a flexure that attaches to an end of the head support arm. Each head includes an interactive element such as a magnetic transducer. The transducer either (1) senses the magnetic transitions on a selected data track to read the data stored on the track, or (2) transmits an electrical signal that induces the magnetic transitions on the selected data track to write data to the data track. Air currents are caused by the high-speed rotation of the discs. A slider assembly included on each head has an air bearing surface that interacts with the air currents to cause the head to fly at a short distance above the data tracks on the disc surface.
In order to improve data access performance of a disc drive, it is generally desirable to maximize the stiffness and to minimize the inertia of the actuator assembly. A stiffer system has a faster response time, because an increase in stiffness reduces the xe2x80x9csettlexe2x80x9d time at the desired read or write location. A stiffer system also minimizes vibrations that cause errors in reading and writing information from the heads to the discs. A lower inertia allows an actuator assembly to be moved quickly from one location to another without excessive power requirements.
Several mechanical properties affect the stiffness and the inertia of a mechanical system, including: (1) material density for the various components of the actuator assembly; (2) flexural rigidity; and (3) specific modulus. For a given specification of physical dimensions, minimizing the material density minimizes the mass of the actuator assembly, which in turn minimizes the inertia. The flexural rigidity, or bending modulus, of a head support arm is the elastic modulus (E) of the material multiplied by the moment of inertia (I) of the head support arm. The specific modulus is the elastic modulus of the material divided by the density of the material. The specific modulus may be thought of as a measure of the stiffness per unit mass of a beam. For the same design, an actuator built with a higher modulus material will have higher critical natural resonances.
In the case of an actuator assembly for a disc drive, there are two types of vibration that one is concerned with: (1) angular vibration caused by rapid pivoting and stopping of pivoting of the actuator assembly; and (2) a transverse vibration normal to the plane of pivoting of the actuator assembly. In the case of transverse vibration, the neutral axis is defined by a horizontal plane at some fixed height along the pivot axis.
The actuator assembly has as its first major vibrational mode in the cross-track direction a mode called the xe2x80x9cbutterfly mode.xe2x80x9d The butterfly mode of vibration refers to an angular vibration that occurs when the coil yoke and the head support arm, in pivoting about the shaft, move toward one another. Ultimately, one is primarily concerned with the vibration of the head support arm, because it supports the read/write heads. However, it is also desirable to minimize the vibration of the coil yoke, because it is mechanically coupled to the head support arm.
For a beam, assuming other properties are constant, the stiffness of the beam increases as I increases, where I is the moment of inertia of a cross-section of the beam about a neutral axis. It has been a well-known principle in the construction industry that an I-beam deflects less than a solid beam of an equivalent mass and an equivalent modulus of elasticity. This is true because an I-beam has a larger moment of inertia than a solid beam. The I-beam has a larger moment of inertia than a solid beam because much of its mass is xe2x80x9cspacedxe2x80x9d away from the neutral axis of the beam. Because the moment of inertia is larger, the I-beam is generally stiffer than the solid beam of an equivalent mass. For any arbitrary beam, according to the beam flexure formula:
"sgr"=My/I, where
"sgr"=a stress at some point in the beam
M=an applied bending moment;
y=a distance from a neutral axis of the beam; and
I=a moment of inertia about the neutral axis.
For a point on the surface of the beam, this formula becomes:
"sgr"max=Mymax/I.=M/c
where
c=I/Ymax=elastic section modulus.
Because of these properties, it is desirable to apply the same principles of I-beam stiffness to configure a head support arm and a coil yoke with a maximum stiffness and a minimum mass.
Thus, there is a need for a geometric configuration for a head support arm and a coil yoke with a relatively large moment of inertia and a relatively low mass.
The present invention is for an actuator assembly having an actuator body, from which project shell extensions. The shell extensions may be head support arms or a coil yoke. Shell structures, or shells, typically have a higher moment of inertia than flat solid structures. A shell is a three-dimensional curved structure with one of its dimensions, a thickness, much smaller than its other two dimensions. Shells are best visualized by first picturing a flat plate of a uniform thickness, and then picturing deforming the plate so that one of its surfaces is concave, one of its surfaces is convex, and the two surfaces are still separated by the thickness of the plate.
An actuator assembly is provided for a computer disc drive. The actuator assembly includes an actuator body that supports a head support arm and a coil yoke. The coil yoke supports the coil of the VCM. The actuator body pivots about a pivot axis and is attached to the cartridge bearing assembly disposed within an inner hollow of the actuator body.
The head support arm is a shell having a core, a first surface and a second surface. The first surface is concave and the second surface is convex. The head support arm is connected to the actuator body at a proximal end of the head support arm. At a distal end of the head support arm, there is a flat portion with a swage hole for connection of the flexure assembly. The coil yoke has a first surface, a second surface, and a coil yoke core separating the two surfaces.